1. Field of the Invention
This invention relates to methods and apparatus for mounting electronic components on printed circuit boards. In particular, surface mounting of circuit board components is effected using a manually operable mounting head, specifically structured such that all the movements of the mounting head necessary to place the components can be accomplished by the user with one hand.
2. Prior Art
Electronic circuits are mounted on printed circuit boards having a pattern of conductors on an insulating surface or substrate. Various electronic components are arranged on the surface such that individual leads of the components correspond in position to the conductors, and can be electrically coupled into a circuit by soldering. Surface mounted devices (or "SMDs") are soldered to conductor pads or "lands" that lay flat on the surface of the printed circuit board. Other types of packages, such as the dual-in-line package, have protruding leads that are intended to extend into holes in the printed circuit board at the lands. In either case, leads of the mounted package are coupled into a circuit by conductors that extend along the surface of the board and may be coupled to additional conductors on the opposite surface or at a different level.
A circuit package having protruding leads must be positioned accurately such that the leads align with the holes in the circuit board, permitting the leads to be inserted. SMDs also must be positioned accurately, so that their conductive leads, generally terminating in small pads around the lower peripheral edge of the SMD, rest on the corresponding lands of the printed circuit board. Such positioning can be demanding for protruding leads, because a missed insertion will bend one or more leads. The positioning is also demanding for SMDs, especially because the pads and lands are very small and very closely spaced, such that a minimal error can cause solder bridges to be formed, shorting between adjacent pads and/or lands.
SMDs have developed in response to a need to mount components and conductors more closely and densely on circuit boards, for compactness and also to shorten the length of conductors between circuit elements for improved speed and electrical properties in the circuit as a whole. The leads of an SMD can terminate on the surface of the SMD package in a so-called "leadless" configurations. In that case conductors to the circuit element or elements in the SMD package are coupled to conductors presented on the surface of the package. SMDs also may have leads that extend outwardly from the surface of the SMD package. However, unlike devices having leads that are inserted into holes in the circuit board, such "leaded" SMD packages simply rest on the surface of the circuit board, where the leads are soldered to lands that correspond in spacing to the leads. For purposes of this disclosure, an SMD device is construed to include a surface mounted receptacle having conductors for coupling width a device received in the receptacle. The receptacle may have other means such as spring biased conductors or clamps for coupling electrically to the device received in the receptacle.
Components can be positioned on a board by hand. However, this is a most laborious and exacting process. The components, such as delicate SMDs, can be damaged by manual handling. To address mounting problems, a variety of automatic and semiautomatic apparatus have been developed to more efficiently assemble circuit boards.
It is known to mount SMDs and the like using a vacuum-assisted mounting head. The head picks up the SMD (for example an integrated circuit with a plurality of leads), and holds the SMD adjacent a vacuum inlet. After moving the head to the required position the vacuum is released, detaching the SMD from the head, hopefully as accurately positioned over the corresponding lands on the circuit board.
Various structures can be used in picking up, placing and releasing an SMD or the like. An apparatus having a single mounting head that is movable between a pick-up position and a placement position is disclosed, for example, in U.S. Pat. No. 4,872,258--Ragard. U.S. Pat. No. 5,093,984--Lape discloses an apparatus arranged to displace a mounting head in a first direction and to displace the circuit board in a direction perpendicular thereto, to effect X-Y positioning of a component relative to the circuit board.
Multi-head devices provide further alternatives for positioning SMDs. A plurality of mounting heads can be arranged around a rotatable turret. A mounting head on the turret is lowered in a direction parallel to the turret rotation axis to engage an SMD at a first position angular position around the turret, then raised and rotated with the turret to a second angular position where the SMD is deposited on the circuit board. A multi-head device of this type is disclosed in U.S. Pat. No. 4,979,286--Nakayama et al. A rotatable multi-head turret also can be combined with a further positioning mechanism for providing X-Y positioning, for example as disclosed in U.S. Pat. No. 4,985,986--Fritsch.
Movement of one of the heads around the turret results in rotation of the device carried in the head. It is necessary to align the SMD or other device rotationally as well as to move the device to the required X-Y location on the board. U.S. Pat. No. 4,979,286--Nakayama et al discloses a mounting head mechanism having a head rotation drive mechanism that rotates a selected mounting head about its own axis so as to angularly position the SMD prior to placement on the circuit board.
Prior art positioning systems for SMDs and similar circuit components generally are complicated and expensive electromechanical robotic devices. Such devices may be suitable for large scale production lines where they can repetitively place components on circuit boards according to a rigid program that defines all the steps and positioning movements needed to place a component on the circuit board. It would be advantageous, however, to provide a positioning mechanism that is more appropriate for smaller and more customized production runs, that is substantially lower in cost, is manually operated, and is capable of versatile and accurate component placement at a reasonable speed. Such a device advantageously would take advantage of the aspect of typical circuits that a number of identical resistors, capacitors or other components are used at various locations on the board, and would be arranged for efficient operation under manual control of an operator, to achieve the greatest possible efficiency of the operator, i.e., to very accurately and inexpensively place the maximum number of components on circuit boards in the minimum amount of time.